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Cross-coupling reactions between two alkenes, leading to new functionalised alkenes, can be brought about via‘hydrocobaltation’ of one of the alkenes followed by irradiation of the resulting organocobalt reagent in the presence of the second alkene substrate.

LiAlH4 reacts with olefins in the presence of a catalytic amount of UCl4 or UCl4 to givbe alkylaluminate compounds. The active species in these reactions is thought to be U(AlH4)3.

Treatment of acetylenes with Ph3SnH or Ph3GeH in the presence of a catalytic amount of Pd(PPh3)4 provides the corresponding alkenyltriphenylstannanes or alkenyltriphenylgermanes in good yields.

The stereoselective elaboration of alkynes into substituted alkenes via vinylic organometallics derived from hydrometalation and carbometalation of the triple bond comprises one of the most powerful tools for olefinic synthesis. The discovery that mono- and disubstituted alkenylaluminum compounds can be directly synthesized via cis hydroalumination of alkynes thus has played an important role in the development of stereoselective syntheses of functionally di- and tri-substituted olefins. Also, the introduction of the zirconium-catalyzed carboalumination has made available alkenylaluminum compounds that can be elaborated into stereodefined trisubstituted olefins, especially those of terpenoid origin. The fact that the hydroalumination of 1-alkynes not only provides alkenylalanes, but also can be controlled through the choice of solvent to afford 1-alkynylalanes and bis- and tris-aluminoalkanes greatly enhances the versatility of alkyne-derived organoaluminum derivatives. However, the chemistry of geminal aluminoalkanes remains to be delineated. An important feature of alkenyl- and alkynylaluminum compounds is that they exhibit chemical properties that differ distinctly from those of the corresponding organolithium or organomagnesium compounds or of their organoboron congeners. For example, although unsaturated organoaluminum compounds undergo Grignard-like reactions with a variety of organic and inorganic electrophiles, they show some significant differences in reactivity and substrate selectivity as compared to organolithium and Grignard reagents. This results because the trigonal aluminum in organoalanes possesses a rather low intrinsic nucleophilicity. However, conversion of the organoalanes into the corresponding filled-octet aluminate species by treatment with bases markedly increases their carbanion character. Also, transmetalation of alkenylaluminum compounds with transition metals produces stereodefined organometallics with reaction characteristics different from those of their progenitors, thus providing an additional tool for modifying the reactivity of unsaturated organoaluminum compounds. For simplicity and clarity, organoaluminum compounds are referred to in this chapter as monomers even though they actually may be associated. Except for the 1-alkynylaluminum compounds, we have chosen to include in this chapter only those alkenylaluminum compounds whose preparations involve the direct formation of a carbon–aluminum bond. Those derived by exchange reactions are not covered. Keywords: alkenylaluminum compounds; alkynylaluminum compounds; hydroalumination; olefins; functionally substituted olefins; dienes; acetylenes; hydroxyacetylenes; enynes; group transfer; conjugated enones; cyclopropanation; transmetalation; alkenes; halogen compounds; allylic alcohols; allylic ethers; acetylenes; cyclopropanes; alkenylsilanes; ketoalkenes; ketoacetylenes; syntheses; experimental procedures

The reagent system generated from Cp2TiCl2(Cp=cyclopentadienyl) and LiAlH4 promoted the catalytic hydrometalation and isomerization of olefins, the catalytic ability of the reagent system for both reactions strongly depending on the composition of two reagents in the reagent system. The hydrometalation occurred in the anti-Markownikoff fashion. By these two catalytic reactions, 1-octene was effectively converted to 1-bromooctane and 2,3-dibromooctane.

In contrast to classically protected ω-hydroxy alkynes, ω-terbutoxy alkynes undergo smooth hydroalumination with DIBAL-H. The obtained E or Z alkenyl alanes are iodinated leading to E or Z alkenyl iodides. The ω-terbutoxy group is easily converted into an acetate. The latter compounds are useful synthons for the synthesis of insect pheromones as examplified with two cases.

Publisher Summary This chapter examines the methods of preparation, properties, stereochemical aspects and typical reactions of carbenes, alkenes, allenes, dienes, and acetylenes or alkynes. Carbenes may be prepared from diazocompounds, cycloeliminations, base-catalyzed α-eliminations, and Simmons-Smith and related methylene transfers. Carbenes undergo typical reactions, such as cycloadditions, insertions, rearrangements, and transition metal complexes. Several powerful methods have been developed for the stereoselective synthesis of alkenes. These include eliminations of HX, dehalogenation of vicinal dihalides, decarboxylation of hydroxy acids, Wittig reaction and related procedures, such as partial reduction, and hydroboration of alkynes. Alkenes undergo various chemical reactions, such as electrophilic additions, addition of halogens and interhalogens, free-radical additions, oxidation, reduction, photochemical reactions, thermal cycloadditions, ene reactions, rearrangements, and metal and metal complex catalyzed reactions. Hydrocarbons containing two double bonds are studied under allenes and dienes; their synthetic methods, properties and typical reactions are analyzed in the chapter. Preparation of acetylenes by dehydrohalogenation methods, metal acetylides, and isomerization of other acetylenes bear specific significance in generation and characteristics of unsaturated acyclic hydrocarbons. Several typical reactions, such as oxidation, reduction, electrophilic addition, such as halogenation, additions of H-X hydrometalation, nucleophilic additions, Diels-Alder cycloadditions, photochemical reactions, cyclo-oligomerization and polymerization are also outlined.

Manganese pentacarbonyl hydride (I) reacts with 1,3-butadiene, isoprene, and 2,3-dimethyl-1,3-butadiene to form predominantly the hydrometalated products of 1,4-addition of H-Mn(CO)/sub 5/ to the dienes, i.e. I + CH/sub 2/=CRCR'=CH/sub 2/ ..-->.. MeCR=CR'CH/sub 2/Mn(CO)/sub 5/ R,R' = H, Me. Monoolefins, the products of stoichiometric 1,2- and 1,4-addition of two hydrogen atoms to the dienes, are also formed as minor products, i.e. 2I + CH/sub 2/=CRCR'=CH/sub 2/ ..-->.. MeCHRCR'=CH/sub 2/ + CH/sub 2/=CRCHR'Me + MeCR=CR'Me + Mn/sub 2/(CO)/sub 10/. Hydrogenation is the predominant process with 1,3-cyclohexadiene. The hydrometalation reactions are first order in both I and diene, and the reaction rates are unaffected by added carbon monoxide (1 atm), while careful NMR monitoring of the early stages of the reactions under appropriate conditions reveals striking CIDNP polarization is for the /sup 1/H resonances of both the products of hydrometalation and of hydrogenation. The experimental evidence thus suggests that the reactions do not proceed via prior coordination of the olefin to the metal followed by conventional migratory insertion and reductive elimination processes but rather via hydrogen atom abstraction from I by the diene to give the corresponding (ally) + Mn(CO)/sub 5/) radical pair.

The cycloisomerization of enynes catalyzed by Pd(OAc)2 and bis-benzylidene ethylenediamine (bbeda) is a landmark methodology in transition-metal-catalyzed cycloisomerization. However, the mechanistic pathway by which this reaction proceeds has remained unclear for several decades. Here we describe mechanistic investigations into this reaction using enynamides, which deliver azacycles with high regio- and stereocontrol. Extensive 1H NMR spectroscopic studies and isotope effects support a palladium(II) hydride-mediated pathway and reveal crucial roles of bbeda, water, and the precise nature of the Pd(OAc)2 pre-catalyst. Computational studies support these mechanistic findings and lead to a clear picture of the origins of the high stereocontrol that can be achieved in this transformation, as well as suggesting a novel mechanism by which hydrometalation proceeds.

Two-step hydrolithiation of 3,4,6-tri-O-benzyl-D-glucal (hydrochlorination and lithium naphthalenide reductive lithiation) gives a reactive glycosyl-lithium derivative which is shown to be a precursor of C-glycosides.

Some higher organozinc compounds were synthesized by reacting ZnH2with linear and cyclic olefins in the presence of a catalyst based on ZrCl4 and C4H9OH.

Copper(I) hydride species, prepared in situ in tetrahydrofuran using NaBH4–MgBr2–Et3N–CuCl or NaH–MgBr2–CuCl reagent systems, convert terminal alkynes into (E,E)-1,3-dienes in good yields.

Irradiation of methyl methacrylate (MMA) in the presence of acyl- and alkyl-cobalt ‘salophen’ reagents leads to good yields of oligomers [(11)–(13)] by way of a novel catalytic ‘hydrocobaltation–dehydrocobaltation’ sequence; hydrocobaltation of the dimer (11)[to (14)] followed by conversion of the latter into (15) in the presence of styrene demonstrates the possibilities for ‘reactivation’ of oligomers of MMA for further chemical studies.